The PEG ratio answers a question the plain P/E ratio can't: is this stock's price justified by how fast its earnings are actually growing? A P/E ratio alone tells you how many dollars investors are paying for each dollar of current earnings, but it says nothing about growth — which means a stock with a sky-high P/E can still be a bargain if its earnings are compounding fast enough, while a stock with a low, seemingly cheap P/E can actually be expensive if its earnings are barely growing at all. PEG was popularized by investor Peter Lynch specifically to catch this blind spot.

Note the growth rate convention: a 20% growth rate is entered as 20, not 0.20, when dividing into the P/E ratio. This is the standard convention used by Peter Lynch and virtually every PEG calculation in practice — get this backwards and the result will be off by a factor of 100.

Consider two stocks that, on P/E alone, look completely different:

Stock A looks far cheaper on P/E alone — a multiple of just 10 versus Stock B's eye-watering 50. But once growth enters the picture, the conclusion flips entirely: Stock A's PEG of 5.0 reveals it's actually quite expensive relative to its near-stagnant 2% growth, while Stock B's PEG of 1.25 — despite the much higher headline P/E — is far closer to "fairly priced" once its much faster 40% growth is accounted for. This is exactly the kind of value trap PEG is designed to catch.

PEG of exactly 1.0 is the traditional rule-of-thumb fair value line, but it's a starting heuristic, not a hard rule — different sectors and market conditions can shift what counts as a reasonable PEG. High-growth technology stocks have historically traded at PEG ratios above 1.0 for extended periods without necessarily being "wrong," while slow-growing utility or industrial stocks are more likely to be judged against the 1.0 benchmark literally.

Using forward estimates versus trailing actuals for the same stock can produce meaningfully different PEG ratios, particularly for companies whose growth is accelerating, decelerating, or recovering from a temporary slump. Neither basis is strictly "correct" — checking both gives a more complete picture than relying on just one.

PEG assumes a roughly linear relationship between valuation multiple and growth rate, which is a simplification — markets don't actually price growth perfectly linearly, and a single ratio can't capture differences in profit margins, balance sheet strength, competitive position, or the quality and sustainability of that growth. A company growing earnings 30% a year by taking on heavy debt is a very different (and riskier) story than one growing the same 30% organically with high margins, even though both could show an identical PEG ratio.